Effect of Maturation on α-Adrenoceptor Activity in Newborn Piglet Mesentery

Abstract

To characterize the mesenteric α1- and α2-adrenoceptor populations in newborn piglets, an extracorporeal circuit was established to control intestinal blood flow in 0- to 2-day old and 10- to 14-day old animals. In both groups, α-adrenoceptor activation was first documented by observing dose-dependent increases in mesenteric perfusion pressure after intramesenteric arterial injection of α-adrenoceptor agonists. In the 10-to 14-day old piglets, mesenteric vasoconstrictor responses to α1-adrenoceptor agonists (methoxamine and norepinephrine) and an α2-adrenoceptor agonist (BHT-933) were each blunted (P< 0.05, analysis of variance) by peripheral intravenous injections of prazosin (an α1-adrenoceptor antagonist) and yohimbine (an α2-adrenoceptor antagonist), respectively. The mesenteric vasoconstrictor responses to those agonists were not significantly attenuated by prazosin or yohimbine in 0- to 2-day old animals, nor were they blunted by YM-12617 (α1-adrenoceptor antagonist) or idazoxan (α2-adrenoceptor antagonist)—compounds that are structurally unrelated to prazosin and yohimbine, respectively. In addition, mesenteric vasoconstrictor responses to other known vasoconstrictor agents—angiotensin II, neuropeptide Y, and a thromboxane A2mimic (U-46619)—were not effected in either age group by prazosin or yohimbine, implying these agents act independently of α-adrenoceptor mechanisms. These data suggest that (1) there exists functional mesenteric α1- and α2-adrenoceptor-like activity in 10- to 14-day old piglets that, in 0- to 2-day old animals, is not specifically expressed; and (2) mesenteric α-adrenoceptor function becomes more selective as newborn piglets mature.